Phase inversion in dispersed liquid-liquid pipe flow

Abstract

This thesis presents the experimental and theoretical investigations on the development of
phase inversion in horizontal pipeline flow of two immiscible liquids. It aims to provide
an understanding on the flow development across the phase inversion transition as well as
the effect on pressure drop.
Experimental investigation on phase inversion and associated phenomena were
conducted in a 38mm I.D. liquid pipeline flow facility available in the Department of
Chemical Engineering at University College London (UCL). Two sets of test pipelines
are constructed using stainless steel and acrylic. The inlet section of the pipeline has also
been designed in two different configurations – (1) Y-junction inlet to allow dispersed
flow to be developed along the pipeline (2) Dispersed inlet to allow formation of
dispersion immediately after the two phases are joined. Pressure drop along the pipeline
is measured using a differential pressure transducer and is studied for changes due to
redistribution of the phases during inversion. Various conductivity probes (ring probes,
wire probes, electrical resistance tomography and dual impedance probe) are installed
along the pipeline to detect the change in phase continuity and distribution as well as
drop size distribution based on the difference in conductivity of the oil and water phases.
During the investigation, the occurrence of phase inversion is firstly investigated and the
gradual transition during the process is identified. The range of phase fraction at which
the transition occurs is determined. The range of phase fraction becomes significantly
narrower when the dispersed inlet is used. The outcome of the investigation also becomes
the basis for subsequent investigation with the addition of glycerol to the water phase to
reduce the interfacial tension. Based on the experimental outcome, the addition of
glycerol does not affect the inversion of the oil phase while enhancing the continuity of
the water phase. As observed experimentally, significant changes in pressure gradient can be observed
particularly during phase inversion. Previous literatures have also reviewed that phase
inversion occurs at the maximum pressure gradient. In a horizontal pipeline, pressure
gradient is primarily caused by the frictional shear on the fluid flow in the pipe and, in
turn, is significantly affected by the fluid viscosities. A study is conducted to investigate
on the phase inversion point by identifying the maximum mixture viscosity (i.e.
maximum pressure gradient) that an oil-in-water (O/W) and water-in-oil (W/O)
dispersion can sustain. It is proposed that the mixture viscosity will not increase further
with an increase in the initial dispersed phase if the inverted dispersion has a lower
mixture viscosity. This hypothesis has been applied across a wide range of liquid-liquid
dispersion with good results. This study however cannot determine the hysteresis effect
which is possibly caused by inhomogeneous inversion in the fluid system.
A mechanistic model is developed to predict the flow characteristics as well as the
pressure gradient during a phase inversion transition. It aims to predict the observed
change in flow pattern from a fully dispersed flow to a dual continuous flow during phase
inversion transition. The existence of the interfacial height provides a selection criterion
to determine whether a momentum balance model for homogeneous flow or a two-fluid
layered flow should be applied to calculate the pressure gradient. A friction factor is also
applied to account for the drag reduction in a dispersed flow. This developed model
shows reasonable results in predicting the switch between flow patterns (i.e. the
boundaries for the phase inversion transition) and the corresponding pressure gradient.
Lastly, computational fluid dynamic (CFD) simulation is applied to identify the key
interphase forces in a dispersed flow. The study has also attempted to test the limitation
of existing interphase force models to densely dispersed flow. From the study, it is found
that the lift force and the turbulent dispersion forces are significant to the phase
distribution in a dispersed flow. It also provides a possible explanation to the observed
flow distribution in the experiments conducted. However, the models available in CFX
are still unable to predict well in a dense dispersion (60% dispersed). This study is also suggested to form the basis for more detailed work in future to optimize the simulation
models to improve the prediction of phase inversion in a CFD simulation.